The present invention relates to a disk brake having a disk rotor and friction pads adapted to be kept in contact with the disk rotor even while the brake is not applied so as to keep off earth, sand or muddy water that may stick to the disk rotor while the vehicle is traveling on an unpaved road and to keep applying a light, stable dragging torque to the disk rotor, thus preventing uneven wear of the disk rotor while the vehicle is traveling on a paved road.
In order to keep off earth, sand or muddy water stuck on the disk rotor of a disk brake while the vehicle is traveling on an unpaved road, some conventional disk brakes have a separate scraper provided near the caliper body. Other conventional disk brakes have their friction pads made from a sintered metal that is insensitive to water fade. The former arrangement is uneconomical because it needs, besides the separate scraper, an adjustor means that follows the wear of the scraper. In the latter arrangement, although water fade lessens, the friction pads made of sintered metal tend to severely attack the disk rotor. Also, their friction coefficient tends to fluctuate according to use conditions. Moreover, it is necessary to increase the vibration-proofness of peripheral parts because the friction pads are heavy.
The disc rotor may be worn unevenly, attacked by friction pads due to run-out of the disk rotor. That is, the run-out of the disk rotor causes the disk rotor to be periodically brought into contact with the friction pads in each revolution of the disk rotor. The disk rotor is thus worn unevenly or locally, developing thin portions circumferentially alternating with thick portions. When the pads are pressed against such an unevenly worn disk rotor to brake the disk while the vehicle is traveling at medium to high speeds, the fluid pressure in the cylinders fluctuates at short intervals. This causes the brake pedal and the steering wheel to judder.
One way to prevent such a judder is to minimize run-out of the disk rotor. Another way is to secure a gap between the disk rotor and the friction pads without impairing the piston retraction function. Such a means is disclosed in Japanese patent publication 9-53668 filed by the applicant of this invention. Specifically, the disk brake disclosed in this application has a shim in the shape of a flat plate disposed between the friction pads and the pistons. The shim couples the friction pads to the pistons. Thus, when the pistons retract, the friction pads also retract together with the pistons out of contact with the disk rotor.
Automobiles travel on various kinds of grounds. RV's may travel not only on paved roads but on muddy or marshy grounds. The disk rotor of a disk brake is exposed to the air, so that it tends to be affected by the environmental changes. For example, earth, sand or muddy water may invade into the gaps between the disk rotor and the friction pads while the vehicle is traveling on an unpaved road, causing fluctuation in the friction coefficient or abnormal wear of sliding parts. It is therefore important to keep the surface of the disk rotor in a stable state.
One way for this aim is to use the friction pads themselves as scrapers for scraping off any foreign matter stuck on the rotor surface. For this purpose, it is important to select such a material for the sliding members and attain such sliding conditions that the wear of the friction members and the energy loss are kept to a minimum.
An object of the present invention is to provide a disk brake having the friction pads kept in contact with the disk rotor to avoid the entry of sand and muddy water and which functions satisfactorily even if they are kept in contact with each other.